Biomedical subjects

J Gjønnes

Publications and source records attributed to J Gjønnes.

4 recordsLinked to original sources

PubMed1992

Bloch wave treatment of symmetry and multiple beam cases in reflection high energy electron diffraction and reflection electron microscopy.

Bloch wave equations for the multiple beam cases in reflection high energy electron diffraction (RHEED) are derived from the integral equation by forward- and back-scattering Green function operators. A linearization is achieved through separation in a forward- and a back-scattering component for each beam. This leads to a set of fundamental equations similar to the transmission case, but with a non-Hermitian matrix, and the beams may be entered as either forward-, back-scattered, or both. The number of beams needed to be included in RHEED calculations is thus reduced, and so are the computing time and computer space required. The systematic row case, corresponding to reflections from planes parallel to the crystal surface, is treated in detail and illustrated by calculations of dispersion surfaces and rocking curves for Au(001). Symmetry relations for the systematic row and between reciprocal rows are discussed and illustrated.

Crystallography↗

PubMed1989

Progress toward structure determination.

Convergent-beam electron diffraction provides more precise measurements of diffracted intensity than the traditional method of selected-area diffraction. The intensity is recorded at well-defined beam directions for each reflection in the pattern within disks defined by the incident cone of rays. Measurements relating to structure factors or parameters can be arranged in different ways: intensities at the zone axis position; Kossel line profiles or integrated intensities across Kossel lines; conditions for vanishing contrast at a Kossel line (e.g., critical voltage); separation between Kossel line segments at intersections. Examples of application to refinement of structure parameters (zone axis intensities) and structure factor determination (Kossel line methods) are given. The relation of these magnitudes to theory is discussed, especially for the Kossel line methods. These are described in terms of effective Fourier potentials or gaps at the Bloch-wave dispersion surface. Use of the methods for refinement of structure parameters and structure factors is reviewed with special attention to recent developments. This is seen along two lines: 1) extended scope for the more accurate methods in order to cover larger unit cells and 2) better precision in measurements of intensities.

Crystallography↗

PubMed1985

Occurrence of airborne silicon carbide fibers during industrial production of silicon carbide.

Airborne dust from the production of silicon carbide has been analyzed for particle morphology and composition. Fibers of alpha silicon carbide were identified by scanning electron microscopy (SEM) combined with energy dispersive X-ray spectrometry (EDS) and transmission electron microscopy (TEM) with selected area electron diffraction techniques (SAED). Micrographs taken at high magnification revealed several stacking periods along the fiber axis, and one or more of the polytypes 2H, 4H, or 6H could be distinguished. Preliminary investigations applying SEM showed that 80% of the fibers had diameters of less than 0.5 micron and a length greater than 5 micron. Fiber concentrations were examined by the counting of stationary and personal samples in an optical phase contrast microscope. The fiber levels in the three plants investigated were low and less than 1 fiber/cc of air (10(6) fibers/m3). Dust samples from the handling of raw material, including recycled material, contained up to 5 fibers/cc (5 X 10(6) fibers/m3).

Air Pollutants, Occupational↗

PubMed1984

Occurrence of fibrous sodium aluminumtetrafluoride particles in potrooms of the primary aluminum industry.

Inorganic fibrous particles have been found in the work environment of the primary aluminum industry. Based on scanning electron microscopic analysis, the concentrations ranged from 9 to 720 fibers/cc. The fibers were generally thinner than 0.1 micron in diameter and shorter than 5 microns. By electron microscopy, energy dispersive X-ray microanalysis, and selected area electron diffraction the fibers were identified as sodium aluminumtetrafluoride.

Air Pollutants, Occupational↗